1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an image carrier included therein and more particularly to a damping member fitted in the image carrier for reducing noise ascribable to the image carrier.
2. Description of the Background Art
It is a common practice with a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus to uniformly charge a photoconductive drum or similar image carrier with a charger. The charger is implemented as either one of a non-contact type charger and a contact type charger.
The non-contact type charger is a corona charger including a charge wire and spaced from the drum. A high voltage is applied to the charge wire for charging the drum by corona discharge. The problem with the corona charger is that it produces ozone, nitrogen oxides and other discharge products that pollute the environment. For this reason, the contact type charger is predominant over the non-contact type charger.
The contact type charger includes a conductive roller, brush, blade or similar charging member held in contact with the drum. A voltage is applied between the charging member and the drum for injecting a charge in the drum. While this type of charger needs only a low voltage and produces no toxic discharge products, the charger is apt to allow toner and impurities deposited on the drum to deposit on the charger. Moreover, when the charging member is left in contact with the drum over a long period of time, part of the charging member contacting the drum is apt to permanently deform. If the charging member contacts the drum in an unexpected manner, then it fails to uniformly charge the drum later.
In light of the above, it has been proposed to form a preselected small gap between the drum and a charging member for thereby forming a non-contact charging range between the drum and the charging member (proximity type charging system). The proximity type charging system, which is the intermediate between the contact type system and the non-contact type system, uses a brush, roller brush, roller blade, belt or similar charging member having adequate conductivity and adequate elasticity. A small gap is formed between the charging member and the drum. An AC-biased DC voltage is applied between the charging member and the drum. For example, when the charging member is implemented as a roller, films with preselected thickness may be wrapped around opposite ends portions of the charge roller, forming the small gap.
The prerequisite with the proximity type charging system is that the small gap has a preselected dimension so as not to vary charging characteristics. More specifically, so long as the small gap has the preselected dimension, a DC voltage easy to set suffices for uniform charging. However, if the gap is greater than preselected one, then the charge potential varies along with the gap. To solve this problem, it has been customary to superpose an AC voltage on a DC voltage for thereby insuring uniform charging even when the gap varies.
The drum includes a conductive core implemented as a hollow cylinder and formed of a relatively light material, e.g., aluminum. This kind of drum sometimes generates noise during image forming operation. More specifically, various units for executing an image forming process, which include charging, optical writing, development, image transfer and cleaning, face the drum. A charger and a cleaning unit, in particular, often constitute the source of noise of the drum.
The AC voltage superposed on the DC voltage, as stated earlier, is likely to cause the thin, hollow cylindrical core of the drum to resonate. Noise ascribable to the resonation is transferred to arrangements around the drum. As for a cleaning unit of the type using a cleaning blade, the cleaning belt held in contact with the drum is repeatedly pulled by the drum in rotation and restored to the original position, vibrating at the thin, cylindrical core of the drum. As a result, the drum resonates and produces noise.
To reduce noise stated above, Japanese Laid-Open Publication No. 7-72641, for example, proposes a photoconductive drum including a solid core, e.g., a cylindrical or columnar, rigid core. However, a problem with this scheme is that the solid metallic core increases the cost and makes the drum extremely heavy. The user is therefore apt to drop the drum or damage the surface of the drum at the time of replacement.
Japanese Patent Laid-Open Publication No. 11-184308 teaches a photoconductive drum in which two or more elastic bodies and cylindrical bodies are fitted. Such sophisticated damping members, however, increase the cost of the drum.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 3-105348 (Japanese Patent No. 2,913,689), 5-35167, 5-197321 and 11-194518.
It is an object of the present invention to provide an image carrier producing a minimum of noise when implemented as a hollow cylinder with a thin wall, and an image forming apparatus using the same.
It is another object of the present invention to provide a damping member for preventing an image carrier from resonating and producing noise.
In accordance with the present invention, an image carrier is included in an image forming apparatus that uniformly charges the photoconductive layer of the image carrier and then electrostatically forms a latent image on the photoconductive layer with a light beam. The image carrier includes a hollow cylindrical core having a thin wall and on which the photoconductive layer is formed. A damping member is fitted in the bore of the core and formed of an elastic material having a loss tangent tan xcex4 of 0.5 or above.